linux-kernel - Re: [PATCH v16 2/4] dmaengine: tegra: Add tegra gpcdma driver

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Date:   Tue, 18 Jan 2022 16:48:23 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Akhil R <akhilrajeev@...dia.com>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Krishna Yarlagadda <kyarlagadda@...dia.com>,
        Laxman Dewangan <ldewangan@...dia.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        "p.zabel@...gutronix.de" <p.zabel@...gutronix.de>,
        Rajesh Gumasta <rgumasta@...dia.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        "vkoul@...nel.org" <vkoul@...nel.org>
Cc:     Pavan Kunapuli <pkunapuli@...dia.com>
Subject: Re: [PATCH v16 2/4] dmaengine: tegra: Add tegra gpcdma driver

18.01.2022 08:36, Akhil R пишет:
>> 17.01.2022 10:02, Akhil R пишет:
>>>> 10.01.2022 19:05, Akhil R пишет:
>>>>> +static int tegra_dma_terminate_all(struct dma_chan *dc)
>>>>> +{
>>>>> +     struct tegra_dma_channel *tdc = to_tegra_dma_chan(dc);
>>>>> +     unsigned long flags;
>>>>> +     LIST_HEAD(head);
>>>>> +     int err;
>>>>> +
>>>>> +     if (tdc->dma_desc) {
>>>>
>>>> Needs locking protection against racing with the interrupt handler.
>>> tegra_dma_stop_client() waits for the in-flight transfer
>>> to complete and prevents any additional transfer to start.
>>> Wouldn't it manage the race? Do you see any potential issue there?
>>
>> You should consider interrupt handler like a process running in a
>> parallel thread. The interrupt handler sets tdc->dma_desc to NULL, hence
>> you'll get NULL dereference in tegra_dma_stop_client().
> 
> Is it better if I remove the below part from tegra_dma_stop_client() so
> that dma_desc is not accessed at all?
> 
> +	wcount = tdc_read(tdc, TEGRA_GPCDMA_CHAN_XFER_COUNT);
> +	tdc->dma_desc->bytes_transferred +=
> +			tdc->dma_desc->bytes_requested - (wcount * 4);
> 
> Because I don't see a point in updating the value there. dma_desc is set
> to NULL in the next step in terminate_all() anyway.

That isn't going help you much because you also can't release DMA
descriptor while interrupt handler still may be running and using that
descriptor.